The wrestling used to be a marvellous English tradition, introduced by Dickie Davies every Saturday at 4.00pm on World of Sport. In church halls and social clubs up and down the country, mad grannies would shake their fists and swing their handbags at figures like Big Daddy, Giant Haystacks and Kendo Nagasaki. We knew it was all fixed, but somehow it didn't matter. It was wonderful. But then the Americans got hold of it and ruined it: they made it look fake. Fighting With My Family is the bogus version of how a girl from Norwich, went over there and make it on the biggest WWE stage.

It's also Stephen Merchant, co-creator of The Office and Extras, first film as a writer-director since the still unexplained break up of his partnership with Ricky Gervais. And it is wildly uneven: parts of it are wonderful, other totally pedestrian. The film is based on a Channel 4 documentary about Paige (Pugh) and her wrestling-mad family whose World Association of Wrestling put on wrestling events around the country for smallish crowds. When the Americans come over to London to audition for talent, her brother (Lowden) is the one that is expected to be selected, but WWE trainer Vaughn goes for her instead.

Most of the scenes in Norwich work a treat. It takes some nerve to cast Frost and Headley as a happily married couple but, after a moment's hesitation, you absolutely believe in the reality of the union of Cersai Lanaster and Simon Pegg's best mate. The mixture of comedy and warmth is just right. All the cast are great. The Rock's cameo may just be him playing himself, but he is extraordinarily good at it. You see what a fantastic creation he is, and how effortlessly he handles everything.

Nearly a decade ago Merchant and Gervais directed and wrote a feature called Cemetery Junction, a tale of working-class kids trying to get out of a dead-end town. It was ok but clunky. It suggested that there was only a really narrow range in which the pair of them could function. Whenever they tried to operate outside of that, their voice became entirely generic. When we're in Norwich Merchant's film is assured and natural, funny and touching. On the other side of the Atlantic the stuff with Vaughn running a WWE boot camp is obvious and cliched; a selection of the same old tired moves.

Though at the beginning it says it is based on true events, clips of the real people shown during the closing credits come as a shock. They undermine your faith in what you have just seen. I know it is now frowned upon to comment on an actress' appearance – because when did looks ever have anything to with a person's ability to be a movie star – but Pugh is short and squat while the real Paige was tall and ripped. Maybe they thought Pugh (excellent in The Falling and Lady Macbeth) was such a talent that it was worth the implausibility, but it distorts the underdog narrative. In the film Paige is this plucky little figure, nothing like the usual WWE lady wrestlers, bravely trying to break into a world dominated by tall willowy blondes. In reality, she was a tall willowy brunette with a funny accent trying to break into that world. I know it's a film about wrestling but it's a bit of a cheat, isn't it?